Viruses are a major biotic constraint on sweet potato (Ipomoea batatas (L.) Lam) production worldwide. In 2005, 10 to 60% viral disease incidence was observed in sweet potato fields. Symptoms include ring and chlorotic spots, puckering, feathering, vein clearing, and leaf curl with chlorotic specks and pink spots. Cuttings from symptomatic plants were collected from Kerala (two clones), Orrisa (eight clones), and Adrapradesh (three clones) and maintained in an insect-proof glasshouse. Leaves from symptomatic plants were mechanically inoculated to I setosa, I. nil, Nicotiana tabacum, N. benthamiana, Datura stramonium, and Chenapodium quinoa (12 seedlings each). Vein clearing, netting, and leaf distortion were observed in I. setosa and N tabacum 7 days postinoculation, chlorotic spots observed in N. benthamiana, and violet spots and violet margins on leaves observed on I. Nil. No symptoms were observed on D. stramonium and C. quinoa. When scions from the symptomatic sweet potato plants were graft inoculated onto I. setosa, vein clearing, leaf curl, and puckering-like symptoms were observed within 5 days. Mosaic and leaf curling symptoms were also observed on mechanically inoculated N. tabacum. Total nucleic acids isolated from the 33 field-collected sweet potato samples, graft inoculated I. setosa plants, and mechanically inoculated N. tabacum and I. nil plants were used for PCR and reverse transcription (RT)-PCR with geminivirus group specific (2) and potyvirus group specific primers (1). The expected 530-bp and 1.3-kb fragment were generated from the geminivirus and potyvirus primer sets, respectively. Potyvirus alone was detected in 7 of the 33 field-collected plants; geminivirus alone was detected in 7 other plants, while 19 plants contained detectible levels of potyvirus and geminivirus. To further identify the viruses, nested primers specific for the coat protein gene of Sweet potato feathery mottle virus (SPFMV) (CP1S 5′AGT GGG AAG GCA CCA TAC ATA GC 3′, CP1A5′ GCA GAG GAT GTC CTA TTG CAC ACC 3′) (CP2S 5′TCT AGT GAA CGT ACT GAA TTC AAA GA 3′, CP2A 5′ATT GCA CAC CCC TGA TTC CTA AGA 3′) and Sweet potato leaf curl virus (SPLCV) (CP1- 5′ATG ACA GGG CGA ATT CGC GTT TC 3′, CP2- 5′TTA ATT TTT GTG CGA ATC ATA 3′) were designed. I. setosa and N. tabacum were amplified with SPFMV and SPLCV primers and the amplicons of 960 and 764 bp, respectively, obtained were subsequently cloned into pGEM-T Easy vector and sequenced. Nucleotide BLAST analysis revealed that the 960-bp fragment (GenBank Accession No. EF015398.) was 98% identical to two Egyptian isolates of SPFMV (Nos. AJ 515379 and AJ 515378). The nucleotide sequence of the 764-bp products (Nos. EF 151926 and EF15483) from the samples collected from Kerala and Orisa was 95% identical to each other. The sequence identity of EF 15483 with Sweet potato leaf curl Georgia virus (SPLCGV) isolate AF326775. was 91% and identity with China isolate DQ 512731 was 90% The isolate EF 151926 also was 91% identical to the SPLCGV with a high query and alignment score whereas identity with the China isolate was 91% with a low query coverage and alignment score. Phylogenic analysis with MEGA software program also showed the highest sequence similarity with SPLCGV, hence it is concluded that the geminivirus isolate under study is SPLCGV. To our knowledge, this is the first report of identification of SPFMV and SPLCGV occurring on sweet potato in India. Further study is required to understand the consequences of occurrence of these two viruses in India.
References: (1) D. Colinet et al. Plant Dis. 28:223 1998. (2) D. D. Deng et al. Ann. Appl. Biol 125:327, 1993.
